The present invention relates to an annular member forming method in which an annular member such as inner and outer races of bearings is formed by cold rolling, and an apparatus for practicing the method (hereinafter referred to as "an annular member forming method", when applicable).
A conventional apparatus and method of forming an annular member have been disclosed, for instance, by Examined Japanese Patent Publication No. Hei. 5-45338.
The conventional annular member forming apparatus, as shown in FIG. 7, essentially includes a mandrel 30 which is rotated at a predetermined position, and a forming roll 31 which is rotated around a shaft which is in parallel with the rotary shaft of the mandrel 30. The forming roll 31 is movable towards and away from the mandrel 30. The forming roll 31 is pressed against an annular workpiece W which has been put on the mandrel 30. Under this condition, the forming roll 31 is rotated and the mandrel 30 is axially rotated with the rotation of the forming roll 31 while the workpiece W is pressed by the mandrel 30 and the forming roll 31 from inside and outside. Hence, the workpiece W is rolled to increase the diameter of the workpiece W. In FIG. 7, a supporting roll 32 supports a load applied to the mandrel 30.
The apparatus further includes a detecting lever 33 which is brought into slide contact with the outer cylindrical surface of the annular workpiece W, and a sensor 34 for detecting the amount of displacement of the detecting lever 33. The outside diameter of the workpiece W which is being rolled is detected by the sensor 34 in cooperation with the detecting lever 33. In the above-described conventional apparatus, the output detection signal of the sensor 34 is applied to a low-pass filter 35, where it is smoothed, and the signal thus smoothed is compared with a predetermined value to detect the completion of the rolling operation, so that the outside diameter of a workpiece W is matched with the average diameter of the workpiece W at the end of the rolling operation.
A conventional annular member forming method using the above-described annular member forming apparatus is described as follows: As shown in FIG. 10, the forming roll 31 is moved towards the mandrel 30 at a high speed in accordance with an instructed amount of depression in rough rolling, the workpiece W is subjected to rough rolling under a predetermined rolling load until the outside diameter of the workpiece W is increased to a predetermined threshold value dc. Next, the forming roll 31 is moved towards the mandrel 30 at a speed lower than the aforementioned high speed (at which the forming roll 31 was moved during rough rolling), and under this condition the workpiece W is subjected to finish rolling under a rolling load smaller than the aforementioned rolling load (under which the workpiece was subjected to rough rolling).
Furthermore, after the workpiece is rolled on the conventional annular member forming apparatus, the workpiece W formed by rolling in the above-described manner is subjected to sizing, as shown in FIG. 8. That is, it is press-fitted into a sizing die 36 to correct the roundness and/or finish dimension of the workpiece W, so that the required workpiece W having high accuracy can be obtained.
A workpiece rolling operation with the conventional annular member forming apparatus will be described in more detail. In the above-described rough rolling operation, as shown in FIGS. 9 and 10, the forming roll 31 is moved towards the mandrel 30 at the predetermined high speed so that the forming roll 31 approaches the mandrel 30, and under this condition the workpiece W is rolled under a high rolling load, so that the workpiece diameter increasing speed is accelerated. In this operation, the annular member forming apparatus is elastically deformed, or elongated, by the high rolling load. Hence, the actual amount of depression of the workpiece W is obtained by subtracting the amount of elongation of the apparatus from the instructed amount of depression. Further, when the outside diameter of the workpiece W reaches the threshold value dc; that is, at the end of the rough rolling operation, the rolling load is a maximum value Pmax.
Under this condition, in order to decrease the amount of depression per revolution of the workpiece W thereby to improve the roundness of the workpiece W, the feeding speed of the forming roll 31 is decreased to switch the rough rolling operation over to a finish rolling operation. At that time, the rolling load is not immediately switched over to a finish rolling load Pf from the aforementioned Pmax. This is because the apparatus elongated in correspondence to the rough rolling load Pmax needs a transient period of time until it is elongated in correspondence to a finish rolling load Pf. After the transient period of time, the rolling load becomes steady, thus reaching the value Pf. Further, when it is detected that the outside diameter of the workpiece reaches a predetermined value df, the rolling operation is ended.
As described above, in the conventional annular member forming operation, the transient period of time is present which is due to the fact that the apparatus is elongated in proportion to the roling load. Hence, there is a time lag in response to the switching from the rough rolling to the finish rolling. That is, the conventional annular member forming operation suffers from a problem that the finishing rolling time is lengthened as much as the above-described transient period of time.
Hence, if the rolling time is shortened; that is, if the finish rolling time is decreased, the rolling of the workpiece may be ended during the transient period of time. In this case, the speed of depression is not sufficiently low yet. Therefore, the amount of depression per revolution of the workpiece W is so large that the resultant workpiece is low in roundness. Hence, it is necessary to subject the workpiece to sizing in the above-described manner, so that the dimension and roundness of the workpiece are corrected to fall within the predetermined allowable ranges.
For instance if the difference between the threshold diameter dc at which the rough rolling operation is switched over to the finish rolling operation and the required outside diameter df of the workpiece is small, then the outside diameter of the workpiece W may reach the required outside diameter df during the transient period of time because the increasing speed of the workpiece diameter is not immediately decreased during the transient period of time.
As is apparent from the above description, in then conventional annular member forming method, an intention of shortening the working time to improve the productivity of the annular member is not compatible with an intention of improving the accuracy of the annular member.